The invention relates generally to terahertz (THz) detectors and, more specifically, to indirect detection THz detectors for use in THz imaging and/or spectroscopy.
Various imaging and/or inspection modalities have been developed for use in a wide range of medical and non-medical applications. For example, in modern healthcare facilities, non-invasive imaging systems are often used for identifying, diagnosing, and treating physical conditions. Currently, a number of modalities exist for medical diagnostic and imaging systems, each typically operating on different physical principles to generate different types of images and information. These modalities include ultrasound systems, computed tomography (CT) systems, X-ray systems (including both conventional and digital or digitized imaging systems), positron emission tomography (PET) systems, single photon emission computed tomography (SPECT) systems, and magnetic resonance (MR) imaging systems. These various imaging systems, with their different respective topologies, are used to create images or views of a patient based on the attenuation of radiation (e.g., X-rays) passing through the patient. Based on the attenuation of the radiation, the topology of the imaging system, and the type and amount of data collected, different views may be constructed, including views showing motion, contrast enhancement, volume reconstructions, two-dimensional images and so forth. Similarly, a wide variety of imaging and/or inspection systems may be utilized in non-medical applications, such as in industrial quality control or in security screening of passenger luggage, packages, and/or cargo. For example, inspection systems are employed at various public or private installations, such as airports, for screening persons, luggage, packages and cargo, to detect the presence of contraband (e.g., weapons, explosives and drugs). Such systems include metal detectors, X-ray based inspection systems, nuclear magnetic resonance based inspection systems, nuclear quadruple resonance based inspection systems, and so forth. In such applications, acquired data and/or generated images may be used to detect objects, shapes or irregularities which are otherwise hidden from visual inspection and which are of interest to the screener. However, these imaging and/or inspection systems have one or more of various limitations such as low reliability in detecting explosives and drugs (leading to high rates of false alarms), health risk to screeners and those being screened due to exposure to harmful radiation, long screening time (leading to decreased throughput at checkpoints), and so forth.
Electromagnetic radiation in the THz range (about 0.1 THz to 10 THz), or THz radiation, or millimeter waves (MMW), is now being used in the field of contraband detection and other applications such as nondestructive testing, medical imaging, dental imaging, multi-spectral imaging and so forth. THz radiation easily penetrates clothes, cardboard, leather and other non-conductive (non-metallic) materials and poses minimal health risk to subjects being scanned. Moreover, a wide variety of contraband, such as explosives, drugs, chemical and biological agents, and so forth, show strong spectroscopic signatures in the THz range. These unique properties offer significant advantages in the field of contraband detection. However, known THz inspection systems are of limited practical utility because of their high cost and limited range of scanning or imaging. Additionally, known THz inspection systems require lengthy scan times per person or per piece of baggage, thereby reducing the throughput and causing inconvenience to those being screened.
For example, current millimeter wave and/or THz systems employ detectors based on expensive high frequency electronics/components, such as monolithic millimeter integrated circuits (MMIC), that are the cost drivers for the entire system. The key components of these detectors are low noise amplifiers (LNAs) built using MMICs having an operating frequency of about 100 gigahertz (GHz). This further limits the use of current THz systems for applications, which require sub-millimeter resolution such as dental imaging. A common strategy for reducing cost is to reduce the number of channels, thereby reducing the number of LNAs. However, reducing the number of channels makes mechanical scanning necessary, thereby increasing maintenance costs and reducing reliability. Thus, current techniques do not allow significant reductions in the number of channels that limits the potential of cost reduction.
Additionally, current THz systems are generally based on passive detection (radiation coming from body itself without being illuminated). Radiation in the millimeter and THz band is weaker than IR radiation and therefore requires higher detector performance and sensitivity with noise equivalent power (NEP) being up to 10−12 Watt/Hertz1/2. Moreover, current THz systems employ detectors based on direct detection principle (sensor directly exposed to the incident radiation) that requires high gain of LNA stages with a flat gain response and gain control of LNA because of oscillations. As noted above, use of expensive LNAs increases the cost of these detectors. Further, known detectors based on indirect detection principles (sensor exposed to parameters that changes based incident radiation), such as helium-cooled bolometers, are costly and have high operating cost.
It is therefore desirable to provide an efficient, reliable, and cost-effective THz detector array working at room temperature for use in a THz inspection or imaging system. It is also desirable to provide an efficient and cost-effective technique for remotely inspecting or imaging a subject using electromagnetic radiation in the THz range.